Structures generate acoustic emissions (AEs) due to development of cracks, friction, or the like inside the structures. AEs are elastic waves that are generated due to development of fatigue cracks of a material. Internal deterioration of a structure can be evaluated by detecting AEs by sensors installed on surfaces of the structure and analyzing signals obtained through the detection. Usually, sensors are adhered to the surfaces of a structure which is to be subjected to degradation evaluation with an adhesive or the like. However, the adhesion of sensors may be insufficient due to defective adhesion work, changes over time, or the like. Such insufficient adhesion may lead to a reduction in the accuracy of deterioration evaluation of the structure or lead to misdiagnosis. Moreover, sensors adhered insufficiently may be at a risk falling off of the surfaces of the structure and thus there is a need to take safety measures.